Microwave waveguide filter

ABSTRACT

A complex waveguide filter which utilizes a number of conventional band-pass cavity resonators in conjunction with pairs of inductively coupled band-stop cavity resonators. The latter being inductively coupled to the waveguide to provide improved passband insertion loss and group delay characteristics.

States Patent Arvind R. Bastikar Kanata, Ontario, Canada App]. No. 5,562 I 7 Filed Jan. 26, 1970 Patented Nov. 2, 11971 Assignee Northern Electric Company Limited Montreal, Quebec, Canada Inventor MienowAvE WAVEGUIDE rrmen 41 Claims, 3 Drawing Figs.

ILLS. Cl 333/73 W lint. Cl "031 3/26, H03h 7/10 lilield of Search 333/70, 73

[56] References Cited UNITED STATES PATENTS 3,451,014 6/1969 Brosnahan et al. 333/73 W 2,432,093 12/1947 Fox 333/73 W 3,221,255 11/1965 Rabowski... 333/241 3,130,380 4/1964 Bowman 333/73 W 2,961,619 11/1960 Breeze 333/73 W Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff Attorney-Curphey & Erickson ABSTRACT: A complex waveguide filter which utilizes a number of conventional band-pass cavity resonators in conjunction with pairs of inductively coupled band-stop cavity resonators. The latter being inductively coupled to the waveguide to provide improved passband insertion loss and group delay characteristics.

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sum 3 OF 3 80 fo-a-Af Hz) BAN D PASS FILTER COMPLEX FILTER l fo-Af (MHz) 80 INVENTOR ARVlND BASTIKAR BY v 626M30 PATENT AGENTS MICROWAVE WAVEGUIDE FILTER FIELD OF THE INVENTION This invention relates to a waveguide filter and more particularly to a balanced band-stop filter which may be used in conjunction with a band-pass filter to provide improved performance using fewer circuit elements.

DESCRIPTION OF THE PRIOR ART In many microwave radio systems, waveguide filters are used as preselectors and diplexers. With high-channel capacity systems, the filters must have a broad passband, high selectivity, low-insertion loss and a small differential or group delay. The former two requirements dictate that the filter have a large number of circuit elements. However, with conventional waveguide band-pass filters, this tends to degrade both the passband insertion loss and the group delay. Thus, if such a filter is located at the front end of a receiver, increased passband insertion loss will increase the noise of the system while a nonlinear group delay will increase the system distortion. In general, the phase delay versus frequency characteristic must be linear over the entire passband in order to avoid distortion problems. Variations in the insertion loss over the passband will also introduce distortion but this is generally not as serious as the group delay.

One common way of obtaining a relatively wide passband and high selectivity is to use a large number of iris-coupled band-pass cavities connected in cascade in a waveguide. However, since the insertion loss and phase delay of such a filter increase with the number of cavities, it has become increasingly difficult to achieve the design specifications required for todays high-performance systems.

SUMMARY OF THE INVENTION It has been discovered that a lesser number of conventional waveguide band-pass filters can be utilized in conjunction with a plurality of balanced band-stop filters in a novel configuration, so that these design parameters can be more readily achieved. While the band-pass and novel band-stop filters can be used in combination as a complex filter to provide improved performance, the novel band-stop filters can also be used alone to provide an improved band-rejection filter.

In accordance with the present invention the microwave filter comprises a waveguide having a plurality of septal irises disposed thereacross at predetermined distances from each other so as to form a number of electrically coupled band-pass cavities of a conventional band-pass filter. In addition, the filter comprises a plurality of pairs of band-stop cavities which are inductively coupled to the waveguide. With rectangular waveguides, the band-stop cavities are inductively coupled through irises along the narrow walls of the waveguide.

One of the inherent problems of coupling cavity resonators to a waveguide is that there is a discontinuity set up at the point of coupling which results in the generation of high-order modes of propagation. These modes are undesirable as they produce interaction between adjacent resonators and deteriorate the electrical performance of the filter. However, the excitation of the even higher order modes has been prevented in the present invention by placing the inductively coupled pairs of band-stop cavity resonators on diametrically opposite sides of the waveguide at the same point along its axis, with resonators of any one pair being substantially identically resonant. The odd higher order modes are suppressed by separating adjacent pairs by an odd number of quarter-wavelengths.

By identically tuning each of the pairs of band-stop cavity resonators at the band edges, the overall selectivity of the sides of the passband can be increased, thus simulating a conventional band-pass filter having additional sections. How ever, because of the relatively high O which is realized using the stop-band cavity resonators, the group delay and insertion loss in the passband is substantially reduced over that which can be obtained using a larger number of cavities in known microwave band-pass filters. As a result, the overall performance of the complex filter is improved. In addition, because of the interaction between cavity resonators in a conventional band-pass filter, it is extremely difficult to optimally tune one having a large number of sections. However, in the present invention, the tuning of the band-pass and the bandstop resonators is completely independent, as the filter can be considered to be of the image parameter type. Since with the present invention, a smaller number of band-pass resonators can be used to achieve the same selectively, the tuning and fabrication of such a filter are simplified.

Additionally, moderate values of 0 may be used in the band-pass filter portion of the complex filter relative to those required for a conventional microwave filter designed to achieve the same electrical performance. Thus, the complex filter of the present invention is a combination of component blocks which may be tuned independently of each other, but which coact to provide an improved performance. The stopband cavities, which are physically located on either side of the passband cavities, contribute very little to the insertion loss or the group delay of the total filter in the passband.

BRIEF DESCRIPTION OF THE DRAWINGS An example embodiment of the invention will now be described with reference to the accompanying drawings in which:

FIG. ii is a perspective view of a complex waveguide bandpass filter in accordance with the present invention;

FIG. 2 is a vertical cross section of the filter taken along the line 2-2 oflFIG. Il; and

FIG. 3 is a graph of insertion loss versus frequency for a typical filter such as is illustrated in FIG. ll.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIGS. 1 and 2, the complex waveguide bandpass filter comprises a waveguide 10 having a four-section band-pass cavity filter section generally 11, which utilizes five septal irises 12 disposed across the waveguide at predetermined distances from each other so as to form a plurality of four interconnected cavity resonators 13. Each of the resonators 13 is tuned to the required frequency by tuning screws 14 in a well-known manner.

In addition, the waveguide filter comprises four pairs of band-stop cavity resonators 15a and 15b. Each of the cavity resonators 15a and llSb is substantially cubic in cross section in order to obtain maximum 0 and hence minimum insertion loss. As shown in FIG. 2, each of the cavity resonators 15a is inductively coupled to the waveguide 110 through irises 16 which are located along the narrow wall of the waveguide 10 and are the full height thereof. A similar set of irises (not shown) is used to couple the cavity resonators 15b to the 0pposite side of the waveguide ill). The width of the iris openings 16 and the number of resonators 1.5a and 15b is determined by the overall filter parameters such as selectivity and group delay.

The width of the iris openings afiect the coupling and hence the loading which in turn determine the notch rejection and the skirt selectivity. In general, the loading can be increased as each of the pairs of resonators llia and 15b is tuned further out from the band edge, without affecting the in-band insertion loss or the group delay. Each of the cavity resonators 15a and 15b is tuned by noncontacting pistons 17 in a well-known manner. By magnetically (inductively) coupling each of the pairs of resonators a and 15b to the narrow wall of the waveguide 10, the disruptive effects on the signals are minimized. This results in advantages for both highand lowpower applications.

If capacitive coupling holes were used in the broad walls of the waveguide 10, they would disrupt the electric field at all frequencies which would result in the: generation of unwanted higher order modes that absorb power and affect the in-band frequency response. In addition, such disruptions may cause arcing in high-power applications.

Each pair of band-stop cavity resonators 15a and 15b is connected to diametrically opposite sides of waveguide and to the same point along its longitudinal axis, at a point remote from the band-pass cavity filter 11. While this symmetry is an asset with inductive coupling, it results in twice as many discontinuities in the waveguide 10. As a result, if the same technique were applied to capacitive coupling, these further disruptions would result in a greater generation of higher unwanted modes. Thus, the invention is limited to inductively coupling the pairs of resonators a and 15b to the waveguide 10.

The cavities of the band-pass cavity filter section 11 are tuned in a conventional manner to have either a Chebishev equal ripple response or a Butterworth maximum flat response. One-half of the band-stop cavity resonators 15a and 15b are tuned to the lower band edge while the other half are tuned to the upper band edge. Additionally, the band-stop cavity resonators of any one of the pairs 15a and 15b are resonant at substantially the same frequency. As a result, even higher order modes are cancelled by the symmetry of the structure.

Each of the pairs 15a and 15b are displaced from each other by an odd number of quarter-wavelengths which (as shown in FIG. 2) is given by the formula (2N--l))./4 where N is any positive integer. Because of the physical dimensions of the band-stop cavities, it was found convenient to locate the pairs of resonators 15a and 15b at five quarters of a wavelength apart. However, other convenient odd number quarterwavelength sections can be readily used. The generation of any higher order modes absorbs power and hence affects both the insertion loss and phase delay. However, because of the balanced arrangement of the band-stop cavities resonators 15a and 15b, the production of high-order modes is minimized. Hence, this efiect on the insertion loss and the group delay by the resonators 15a and 15b is virtually eliminated.

FIG. 3 illustrates a typical frequency response curve of a conventional band-pass filter as well as the complex filter described in FIGS. 1 and 2. Note the nonlinear attenuation scale which is shown thus to emphasize the improved insertion-loss characteristics across the passband. The effect of the band-stop cavities 15a and 15b on the response is noticeable at the band edges. In addition, because the number of bandpass cavities 13 required to obtain the same rejection at the band edges is smaller, the variation in attenuation across the passband is substantially reduced. Also, because of the fewer number of band-pass cavities 13, required to obtain the same selectivity at the band edges, the group delay across the passband is smaller.

What is claimed is:

l. A waveguide filter comprising:

a waveguide;

a plurality of septal irises disposed across the waveguide at predetermined distances from each other so as to form a plurality of electrically coupled band-pass cavity resonators which coact together to provide a band-pass characteristic;

a plurality of pairs of band-stop cavity resonators inductively coupled to the waveguide and axially displaced from each other by an odd number of quarter-wavelengths; the band-stop cavity resonators of each of said pairs being connected to diametrically opposite sides of the waveguide at the same point along the waveguide axis which is remote from the band-pass cavity resonators; and

the band-stop cavity resonators of any one pair being resonant at substantially the same frequency.

2. A waveguide filter as defined in claim 1 in which:

the waveguide is rectangular, and in which the band-stop cavity resonators are coupled to the narrow sides of the rectangular waveguide.

3. A waveguide filter comprising:

a wave uidea plura ity of pairs of band-stop cavity resonators inductively coupled to the waveguide and axially displaced from each other by an odd number of quarter-wavelengths;

the band-stop cavity resonators of each of said pairs being connected to diametrically opposite sides of the waveguide at the same point along the waveguide axis; and

the band-stop cavity resonators of any one pair being resonant at substantially the same frequency.

4. A waveguide filter as defined in claim 3 in which:

the waveguide is rectangular, and in which the band-stop cavity resonators are coupled to the narrow sides of the rectangular waveguide. 

1. A waveguide filter comprising: a waveguide; a plurality of septal irises disposed across the waveguide at prEdetermined distances from each other so as to form a plurality of electrically coupled band-pass cavity resonators which coact together to provide a band-pass characteristic; a plurality of pairs of band-stop cavity resonators inductively coupled to the waveguide and axially displaced from each other by an odd number of quarter-wavelengths; the band-stop cavity resonators of each of said pairs being connected to diametrically opposite sides of the waveguide at the same point along the waveguide axis which is remote from the band-pass cavity resonators; and the band-stop cavity resonators of any one pair being resonant at substantially the same frequency.
 2. A waveguide filter as defined in claim 1 in which: the waveguide is rectangular, and in which the band-stop cavity resonators are coupled to the narrow sides of the rectangular waveguide.
 3. A waveguide filter comprising: a waveguide; a plurality of pairs of band-stop cavity resonators inductively coupled to the waveguide and axially displaced from each other by an odd number of quarter-wavelengths; the band-stop cavity resonators of each of said pairs being connected to diametrically opposite sides of the waveguide at the same point along the waveguide axis; and the band-stop cavity resonators of any one pair being resonant at substantially the same frequency.
 4. A waveguide filter as defined in claim 3 in which: the waveguide is rectangular, and in which the band-stop cavity resonators are coupled to the narrow sides of the rectangular waveguide. 